U.S. patent application number 09/039567 was filed with the patent office on 2002-07-18 for optical transmission device and optical communication system.
This patent application is currently assigned to FUJITSU LIMITED. Invention is credited to KAWASAKI, YUMIKO, NISHIMOTO, HIROSHI, OKANO, SATORU, TSUDA, TAKASHI, YAMANE, KAZUO.
Application Number | 20020093705 09/039567 |
Document ID | / |
Family ID | 17687097 |
Filed Date | 2002-07-18 |
United States Patent
Application |
20020093705 |
Kind Code |
A1 |
OKANO, SATORU ; et
al. |
July 18, 2002 |
OPTICAL TRANSMISSION DEVICE AND OPTICAL COMMUNICATION SYSTEM
Abstract
The present invention relates to an optical transmission device
and an optical communication system applied to wavelength division
multiplexing (WDM). The optical transmission device includes an
optical multiplexer for wavelength division multiplexing a
plurality of optical signals to generate WDM signal light and
outputting the WDM signal light to an optical transmission line, a
detecting unit for detecting a break of each optical signal
according to the power of each optical signal, and a compensator
for adding light having a predetermined wavelength to the WDM
signal light when at least one of the optical signals is cut off.
Through the structure of the present invention it becomes possible
to prevent a deterioration in transmission quality in the case that
the number of WDM channels is changed.
Inventors: |
OKANO, SATORU;
(KAWASAKI-SHI, JP) ; NISHIMOTO, HIROSHI;
(KAWASAKI-SHI, JP) ; YAMANE, KAZUO; (KAWASAKI-SHI,
JP) ; TSUDA, TAKASHI; (KAWASAKI-SHI, JP) ;
KAWASAKI, YUMIKO; (KAWASAKI-SHI, JP) |
Correspondence
Address: |
STAAS & HALSEY LLP
700 11TH STREET, NW
SUITE 500
WASHINGTON
DC
20001
US
|
Assignee: |
FUJITSU LIMITED
Kawasaki
JP
|
Family ID: |
17687097 |
Appl. No.: |
09/039567 |
Filed: |
March 16, 1998 |
Current U.S.
Class: |
398/37 ; 398/34;
398/7; 398/97 |
Current CPC
Class: |
H04B 10/506 20130101;
H04B 10/07955 20130101; H04J 14/0221 20130101; H04B 10/077
20130101; H04B 10/2942 20130101 |
Class at
Publication: |
359/124 ;
359/161 |
International
Class: |
H04J 014/02; H04B
010/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 17, 1997 |
JP |
09-285097 |
Claims
What is claimed is:
1. An optical transmission device comprising: an optical
multiplexer for wavelength division multiplexing a plurality of
optical signals to generate WDM signal light and outputting said
WDM signal light to an optical transmission line; means for
detecting a break of each of said plurality of optical signals
according to the power of each optical signal; and compensating
means for adding light having a predetermined wavelength to said
WDM signal light when at least one of said plurality of optical
signals is cut off.
2. An optical transmission device according to claim 1, further
comprising a plurality of optical senders for outputting said
plurality of optical signals, respectively.
3. An optical transmission device according to claim 1, further
comprising a light source for outputting said light having the
predetermined wavelength, wherein said compensating means comprises
means for switching on/off said light source.
4. An optical transmission device according to claim 1, wherein
said compensating means comprises means for switching on/off light
supplied from an external light source.
5. An optical transmission device according to claim 1, further
comprising: a plurality of optical senders for outputting a
plurality of original optical signals, respectively; and a
plurality of wavelength converters for wavelength converting said
plurality of original optical signals into said plurality of
optical signals, respectively.
6. An optical transmission device according to claim 5, wherein:
each of said plurality of wavelength converters comprises an O/E
converter for converting the corresponding original optical signal
into an electrical signal, and an E/O converter for converting said
electrical signal into the corresponding optical signal; and said
compensating means comprises means for switching on/off said E/O
converter in any inoperative one of said plurality of wavelength
converters.
7. An optical transmission device according to claim 1, further
comprising means for adjusting the power of said light having the
predetermined wavelength.
8. An optical communication system comprising: an optical fiber
transmission line; an optical multiplexer for wavelength division
multiplexing a plurality of optical signals to generate WDM signal
light and outputting said WDM signal light to said optical fiber
transmission line; means for detecting a break of each of said
plurality of optical signals according to the power of each optical
signal; and compensating means for adding light having a
predetermined wavelength to said WDM signal light when at least one
of said plurality of optical signals is cut off.
9. An optical communication system according to claim 8, further
comprising at least one optical amplifier provided in said optical
fiber transmission line for amplifying said WDM signal light.
10. An optical communication system according to claim 9, wherein
said optical amplifier includes a feedback loop for performing
control such that a total output level of said optical amplifier is
maintained constant.
11. An optical communication system according to claim 10, further
comprising means for controlling the power of said light having the
predetermined wavelength so that an output level of said optical
amplifier in each channel is maintained constant.
12. An optical communication system according to claim 9, wherein:
said optical amplifier comprises an optical amplifying medium and
means for pumping said optical amplifying medium so that said
optical amplifying medium provides a gain band including the
wavelengths of said WDM signal light; said predetermined wavelength
being included in said gain band.
13. An optical communication system according to claim 12, wherein:
said optical amplifying medium comprises a doped fiber doped with a
rare earth element; and said pumping means comprises a pumping
source for supplying pump light to said doped fiber.
14. An optical transmission device comprising: a plurality of
optical senders for outputting a plurality of optical signals
having different wavelengths; an optical multiplexer for wavelength
division multiplexing said plurality of optical signals to generate
WDM signal light and outputting said WDM signal light to an optical
transmission line; and at least one light source for adding light
having a predetermined wavelength to said WDM signal light.
15. An optical transmission device according to claim 14, further
comprising means for adjusting the power of said light having the
predetermined wavelength.
16. An optical communication system comprising: an optical fiber
transmission line; a plurality of optical senders for outputting a
plurality of optical signals having different wavelengths; an
optical multiplexer for wavelength division multiplexing said
plurality of optical signals to generate WDM signal light and
outputting said WDM signal light to said optical fiber transmission
line; and at least one light source for adding light having a
predetermined wavelength to said WDM signal light.
17. An optical communication system according to claim 16, further
comprising at least one optical amplifier provided in said optical
fiber transmission line for amplifying said WDM signal light.
18. An optical communication system according to claim 17, wherein
said optical amplifier includes a feedback loop for performing
control such that a total output level of said optical amplifier is
maintained constant.
19. An optical communication system according to claim 18, further
comprising means for controlling the power of said light having the
predetermined wavelength so that an output level of said optical
amplifier in each channel is maintained constant.
20. An optical communication system according to claim 17, wherein:
said optical amplifier comprises an optical amplifying medium and
means for pumping said optical amplifying medium so that said
optical amplifying medium provides a gain band including the
wavelengths of said WDM signal light; said predetermined wavelength
being included in said gain band.
21. An optical communication system according to claim 20, wherein:
said optical amplifying medium comprises a doped fiber doped with a
rare earth element; and said pumping means comprises a pumping
source for supplying pump light to said doped fiber.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to wavelength
division multiplexing (WDM) using a plurality of optical signals
having different wavelengths, and more particularly to an optical
transmission device and an optical communication system applied to
WDM.
[0003] 2. Description of the Related Art
[0004] In recent years, a manufacturing technique and using
technique for a low-loss (e.g., 0.2 dB/km) optical fiber have been
established, and an optical communication system using the optical
fiber as a transmission line has been put to practical use.
Further, to compensate for losses in the optical fiber and thereby
allow long-haul transmission, an optical amplifier for amplifying
signal light has been put to practical use.
[0005] An optical amplifier known in the art comprises an optical
amplifying medium to which signal light to be amplified is supplied
and means for pumping (exciting) the optical amplifying medium so
that the optical amplifying medium provides a gain band including
the wavelength of the signal light. For example, an erbium doped
fiber amplifier (EDFA) comprises an erbium doped fiber (EDF) as the
optical amplifying medium and a pumping source for supplying pump
light having a predetermined wavelength to the EDF. By
preliminarily setting the wavelength of the pump light within a
0.98 .mu.m band (0.97 .mu.m to 0.99 .mu.m) or a 1.48 .mu.m band
(1.47 .mu.m to 1.49 .mu.m), a gain band including a wavelength of
1.55 .mu.m can be obtained. Further, another type optical amplifier
having a semiconductor chip as the optical amplifying medium is
also known. In this case, the pumping is performed by injecting an
electric current into the semiconductor chip.
[0006] As a technique for increasing a transmission capacity by a
single optical fiber, wavelength division multiplexing (WDM) is
known. In a system adopting WDM, a plurality of optical carriers
having different wavelengths are used. The plural optical carriers
are individually modulated to thereby obtain a plurality of optical
signals, which are wavelength division multiplexed by an optical
multiplexer to obtain WDM signal light, which is output to an
optical fiber transmission line. On the receiving side, the WDM
signal light received is separated into individual optical signals
by an optical demultiplexer, and transmitted data is reproduced
according to each optical signal. Accordingly, by applying WDM, the
transmission capacity in a single optical fiber can be increased
according to the number of WDM channels.
[0007] Accordingly, by combining an optical amplifier and WDM, the
span and capacity of an optical communication system can be
increased.
[0008] In the case of combining an optical amplifier and WDM, there
is a possibility that a transmission quality may be deteriorated by
automatic output level control (ALC) performed in the optical
amplifier. In general, ALC is control such that a total output
level of an optical amplifier is maintained constant. Accordingly,
when an optical signal in a certain one of WDM channels is cut off,
for example, an optical output level in each of the other channels
increases to cause a possibility that the transmission quality may
be influenced by nonlinear effects (SPM: Self-Phase Modulation,
XPM: Cross-Phase Modulation, FWM: Four-Wave Mixing, etc.) occurring
in an optical fiber transmission line. It is known that the
influence of nonlinear effects is remarkable particularly in the
case of high-speed transmission at 10 Gb/s or higher.
SUMMARY OF THE INVENTION
[0009] It is therefore an object of the present invention to
provide an optical transmission device and an optical communication
system which can eliminate the possibility of deterioration in
transmission quality in the case that the number of WDM channels is
changed.
[0010] Other objects of the present invention will become apparent
from the following description.
[0011] In accordance with an aspect of the present invention, there
is provided an optical transmission device comprising an optical
multiplexer for wavelength division multiplexing a plurality of
optical signals to generate WDM signal light and outputting the WDM
signal light to an optical transmission line; means for detecting a
break of each of the plurality of optical signals according to the
power of each optical signal; and compensating means for adding
light having a predetermined wavelength to the WDM signal light
when at least one of the plurality of optical signals is cut
off.
[0012] With this configuration, when at least one of the WDM
channels is cut off, the light having the predetermined wavelength
is added to the WDM signal light. Accordingly, in the case of
carrying out ALC (automatic output level control) for maintaining
constant a total output level of an optical amplifier for
amplifying the WDM signal light, for example, a change in optical
output level per channel can be suppressed, thereby achieving one
of the objects of the present invention.
[0013] In accordance with another aspect of the present invention,
there is provided an optical transmission device comprising a
plurality of optical senders for outputting a plurality of optical
signals having different wavelengths; an optical multiplexer for
wavelength division multiplexing the plurality of optical signals
to generate WDM signal light and outputting said WDM signal light
to an optical transmission line; and at least one light source for
adding light having a predetermined wavelength to the WDM signal
light.
[0014] In the case that ALC for maintaining a total output level
constant is carried out in an optical amplifier, the range of
variation in optical output level per channel is dependent on the
number of channels. For example, when the number of channels
decreases from 2 to 1, an optical output level of 3 dB is
increased. In contrast therewith, when the number of channels
decreases from 8 to 7, a change in optical output level is as small
as 0.58 dB. Accordingly, by adding the light having the
predetermined wavelength to the WDM signal light according to the
present invention, the range of variation in optical output level
in each of the remaining channels can be suppressed in the case
that an optical signal in a certain one of the WDM channels is cut
off.
[0015] In accordance with a further aspect of the present
invention, there is provided an optical communication system. This
system includes a terminal station apparatus for transmission and
an optical fiber transmission line operatively connected to the
terminal station apparatus. The terminal station apparatus includes
the optical transmission device according to the present
invention.
[0016] In this specification, the wording that an element and
another element are operatively connected includes the case that
these elements are directly connected, and also includes the case
that these elements are so provided as to be related with each
other to such an extent that an electrical signal or an optical
signal can be mutually transferred between these elements.
[0017] The above and other objects, features and advantages of the
present invention and the manner of realizing them will become more
apparent, and the invention itself will best be understood from a
study of the following description and appended claims with
reference to the attached drawings showing some preferred
embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a block diagram of an optical communication system
to which the present invention is applicable;
[0019] FIG. 2 is a block diagram of another optical communication
system to which the present invention is applicable;
[0020] FIG. 3 is a power diagram for illustrating an allowable
range of transmitted light output;
[0021] FIG. 4 is a block diagram of an optical amplifier applicable
to the present invention;
[0022] FIG. 5 is a graph for illustrating an increase in optical
output power per channel due to a decrease in number of
channels;
[0023] FIG. 6 is a graph for illustrating a decrease in optical
output power per channel due to an increase in number of
channels;
[0024] FIG. 7 is a block diagram showing a first preferred
embodiment of the optical transmission device according to the
present invention;
[0025] FIG. 8 is a block diagram showing a second preferred
embodiment of the optical transmission device according to the
present invention;
[0026] FIG. 9 is a block diagram showing a third preferred
embodiment of the optical transmission device according to the
present invention;
[0027] FIG. 10 is a block diagram showing a preferred embodiment of
a channel number monitor applicable to the present invention;
[0028] FIG. 11 is a block diagram showing another preferred
embodiment of the channel number monitor applicable to the present
invention;
[0029] FIG. 12 is a block diagram showing a fourth preferred
embodiment of the optical transmission device according to the
present invention;
[0030] FIGS. 13A and 13B are block diagrams showing preferred
embodiments of a wavelength converter applicable to the present
invention;
[0031] FIG. 14 is a block diagram showing a fifth preferred
embodiment of the optical transmission device according to the
present invention;
[0032] FIG. 15 is a block diagram showing a sixth preferred
embodiment of the optical transmission device according to the
present invention, and
[0033] FIG. 16 is a block diagram showing a seventh preferred
embodiment of the optical transmission device according to the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0034] Some preferred embodiments of the present invention will now
be described in detail with reference to the attached drawings.
[0035] FIG. 1 is a block diagram of an optical communication system
to which the present invention is applicable. This system includes
a first terminal station 2 for transmitting, a second terminal
station 4 for receiving, and an optical fiber transmission line 6
for connecting the terminal stations 2 and 4. The optical
transmission device according to the present invention is
applicable particularly to the first terminal station 2.
[0036] The first terminal station 2 has a plurality of optical
senders (OS) 8 (#1 to #n) for outputting optical signals (original
optical signals) having arbitrary wavelengths, and a transponder 10
operatively connected to the optical senders 8 (#1 to #n) and the
optical fiber transmission line 6. The transponder 10 includes a
plurality of wavelength converters 12 (#1 to #n) for wavelength
converting the optical signals having arbitrary wavelengths from
the optical senders 8 (#1 to #n) into optical signals having
predetermined wavelengths .lambda..sub.1 to .lambda..sub.n,
respectively, and an optical multiplexer 14 for wavelength division
multiplexing the optical signals from the wavelength converters 12
(#1 to #n) to generate WDM signal light. The WDM signal light from
the optical multiplexer 14 is output to the optical fiber
transmission line 6.
[0037] A plurality of in-line optical amplifiers 16 are provided in
the optical fiber transmission line 6. Each optical amplifier 16
amplifies the WDM signal light. That is, each optical amplifier 16
provides a gain band including the wavelengths of the WDM signal
light. Depending on the length of the optical fiber transmission
line 6, a single optical amplifier may be used.
[0038] The second terminal station 4 has a transponder 18 for
separating the WDM signal light transmitted by the optical fiber
transmission line 6 into individual optical signals (original
optical signals) having arbitrary wavelengths, and a plurality of
optical receivers (OR) 20 (#1 to #n) for receiving these optical
signals, respectively. The transponder 18 includes an optical
demultiplexer 22 for separating the input WDM signal light into a
plurality of optical signals having wavelengths .lambda..sub.1 to
.lambda..sub.n, and a plurality of wavelength converters 24 (#1 to
#n) for wavelength converting these optical signals into optical
signals having arbitrary wavelengths.
[0039] According to the configuration shown in FIG. 1, a
transmission capacity can be increased according to the number of
wavelengths because wavelength division multiplexing is applied.
Further, the span of the optical fiber transmission line 6 can be
increased with a simple configuration because the in-line optical
amplifiers 16 are provided in the optical fiber transmission line
6. In particular, the wavelengths of the optical signals on the
input side of the transponder 10 are arbitrary in the first
terminal station 2, and the wavelengths of the optical signals on
the output side of the transponder 18 are arbitrary in the second
terminal station 4. Accordingly, existing devices may be used both
as the optical senders 8 (#1 to #n) and as the optical receivers 20
(#1 to #n).
[0040] An extended optical network system using a regenerative
repeater instead of each of the optical senders 8 (#1 to #n) or the
optical receivers 20 (#1 to #n) may be provided.
[0041] FIG. 2 is a block diagram of another optical communication
system to which the present invention is applicable. In this
system, a first terminal station 2' for transmission has a
plurality of optical senders 8' (#1 to #n) for outputting optical
signals having predetermined wavelengths .lambda..sub.1 to
.lambda..sub.n for wavelength division multiplexing, respectively.
These optical signals are wavelength division multiplexed by an
optical multiplexer (MUX) 14, and resultant WDM signal light is
output to an optical fiber transmission line 6. In a second
terminal station 41, the WDM signal light transmitted by the
optical fiber transmission line 6 is separated into individual
optical signals having wavelengths .lambda..sub.1 to .lambda..sub.n
by an optical demultiplexer (DMUX) 18, and these optical signals
are supplied to a plurality of optical receivers 20' (#1 to #n),
respectively.
[0042] According also to the system configuration shown in FIG. 2,
an increase in transmission capacity and an increase in span of the
optical fiber transmission line 6 can be achieved like the
configuration shown in FIG. 1. Particularly in the system
configuration of FIG. 2, the optical senders 8' (#1 to #n) and the
optical receivers 20' (#1 to #n) are limited to devices dedicated
for wavelength division multiplexing. However, it is possible to
provide a system with a simpler configuration at a lower cost
because any wavelength converters as shown in FIG. 1 are not
required.
[0043] In each of the systems shown in FIGS. 1 and 2, another
optical amplifier as a postamplifier may be provided between the
optical multiplexer 14 and the optical fiber transmission line 6.
Furthermore, another optical amplifier as a preamplifier may be
provided between the optical fiber transmission line 6 and the
optical demultiplexer 18.
[0044] FIG. 3 is a power diagram for illustrating an allowable
range of transmitted light output. In FIG. 3, the vertical axis
represents optical level (optical power), and the horizontal axis
represents distance (or position). The allowable range of
transmitted light output at the input of an optical fiber span
between two optical amplifiers is determined by a minimum received
power (S/N limit) at the receiving optical amplifier and nonlinear
effects in the optical fiber span. More specifically, the lower
limit of the allowable range of transmitted light output is
determined by a minimum received power and transmission line loss
tilt, and the upper limit of the allowable range is determined by
nonlinear effects. Further, the lower limit of a receivable range
at the receiving optical amplifier is given by the minimum received
power, and the upper limit of the receivable range is determined by
the immunity of a photodetector such as a photodiode.
[0045] Accordingly, in the case that wavelength division
multiplexing is applied in the system as shown in FIG. 1 or 2, it
is important to make the optical levels of the optical signals in
all the channels fall within the allowable range of transmitted
light output in each optical amplifier for the purpose of
maintaining a good transmission quality.
[0046] FIG. 4 is a block diagram of an optical amplifier applicable
to the present invention. The optical amplifier includes an optical
attenuator 30 having a variable attenuation, an optical coupler 32,
a WDM coupler 34, an erbium doped fiber (EDF) 36, and an optical
coupler 38 provided in this order in terms of a propagation
direction of WDM signal light between an input port 26 and an
output port 28. The WDM signal light supplied to the input port 26
undergoes controlled attenuation by the optical attenuator 30 and
is supplied through the optical coupler 32 and the WDM coupler 34
to the EDF 36. Pump light from a laser diode (LD) 40 as a pumping
source is supplied through the WDM coupler 34 to the EDF 36. When
the WDM signal light is supplied to the EDF 36 being pumped by the
pump light, the WDM signal light is amplified in accordance with
the principle of stimulated emission, and the WDM signal light
amplified is passed through the optical coupler 38 and output from
the output port 38.
[0047] In this optical amplifier, automatic gain control (AGC) is
adopted to maintain a gain characteristic (wavelength dependence of
gain) in the EDF 36 constant, and automatic output level control
(ALC) is adopted to maintain a total output level constant.
[0048] The WDM signal light to be supplied to the EDF 36 and
amplified therein is branched by the optical coupler 32, and
resultant branch light is converted into an electrical signal
according to the optical power by a photodetector (PD) 42 such as a
photodiode. Further, the amplified WDM signal light is branched by
the optical coupler 38, and resultant branch light is converted
into an electrical signal according to the optical power by a
photodetector 44. Output signals from the photodetectors 42 and 44
are supplied to an AGC circuit 46. The AGC circuit 46 controls a
drive current to be supplied from a drive circuit 48 to the laser
diode 40 so that the gain of the EDF 36 becomes constant. The
output signal from the photodetector 44 is supplied also to an ALC
circuit 50. The ALC circuit 50 controls the attenuation of the
optical attenuator 30 so that the total output level of the
amplified WDM signal light from the EDF 36 becomes constant.
[0049] While the configuration shown in FIG. 4 adopts forward
pumping such that the WDM signal light and the pump light propagate
in the same direction in the EDF 36, the configuration may be
modified to perform backward pumping such that the WDM signal light
and the pump light propagate in opposite directions in the EDF 36.
Alternatively, bidirectional pumping may be performed by using two
pumping sources.
[0050] In the case that the wavelengths of the WDM signal light to
be amplified are included in a 1.55 .mu.m band (1.50 .mu.m to 1.60
.mu.m), a substantially flat gain band including the wavelengths of
the WDM signal light can be provided by setting the wavelength of
the pump light within a 0.98 .mu.m band or a 1.48 .mu.m band and
properly setting a target value for AGC. Further, high-density
wavelength division multiplexing can be achieved by sufficiently
narrowing wavelength spacings.
[0051] While the EDF 36 is used as an optical amplifying medium in
this optical amplifier, a doped fiber doped with another rare earth
element such as Yb or Nd.
[0052] In the case of carrying out ALC for maintaining the total
output level constant as shown in FIG. 4, there is a possibility
that a change in number of channels of WDM signal light may cause a
change in optical output power per channel to such an extent that
the optical output power deviates from an allowable range of
transmitted light output. This will now be described more
specifically.
[0053] FIG. 5 is a graph for illustrating an increase in optical
output power per channel due to a decrease in number of WDM
channels. It is assumed that WDM signal light of four channels
having a fixed input level is input into an optical amplifier as
shown in a left upper portion of FIG. 5. In the case that an
optical signal in one of the four channels is cut off as shown in a
right upper portion of FIG. 5, an output spectrum corresponding to
the input spectrum, as shown in a left lower portion of FIG. 5
changes into a spectrum shown in a right lower portion of FIG. 5.
That is, the output level of the optical amplifier is controlled by
ALC so that the total output level is maintained constant.
Accordingly, when an optical signal in one channel is cut off, the
output level in each of the remaining channels may exceed the upper
limit of the allowable range of transmitted optical output. In this
case, waveform distortion due to nonlinear effects may occur to
cause a degradation in transmission quality.
[0054] FIG. 6 is a graph for illustrating a decrease in optical
output power per channel due to an increase in number of WDM
channels. It is assumed that WDM signal light of three channels
having a fixed level is input into an optical amplifier as shown in
a left upper portion of FIG. 6. In the case that an optical signal
in one channel is added as shown in a right upper portion of FIG.
6, an output spectrum corresponding to the input spectrum, as shown
in a left lower portion of FIG. 6 changes into a spectrum shown in
a right lower portion of FIG. 6. That is, ALC is performed to
control the output level so that the total output level is
maintained constant. Accordingly, when an optical signal in one
channel is added, the output level in each channel may fall below
the lower limit of the allowable range of transmitted light output.
In this case, the optical power of the optical signal in each
channel on the receiving side may become lower than a minimum
received power, causing a deterioration in transmission
quality.
[0055] There will now be described some preferred embodiments of
the optical transmission device for eliminating the increase in
optical output power per channel as described with reference to
FIG. 5 or for eliminating the change in optical output power per
channel as described with reference to FIGS. 5 and 6.
[0056] FIG. 7 is a block diagram showing a first preferred
embodiment of the optical transmission device according to the
present invention. In this preferred embodiment, the optical
transmission device is applied to the first terminal station 2'
shown in FIG. 2. In this preferred embodiment, the optical
multiplexer 14 has a plurality of input ports respectively
connected to the plural optical senders 8' (#1 to #n) for
outputting optical signals having different wavelengths, and an
additional one input port connected to an additional light source
52. The light source 52 is turned on or off by a control circuit 54
so as to output light having a predetermined wavelength in
accordance with a predetermined rule. For example, when all the
operative channels are in normal operation, the light source 52 is
turned off, whereas when it is detected that an optical signal in
one of the operative channels has been cut off, the light source 52
is turned on. Accordingly, in the case of carrying out ALC such
that the total output power in each optical amplifier 16 shown in
FIG. 2 becomes constant, the optical output power per channel in
each optical amplifier 16 is prevented from being changed to
thereby maintain a good transmission quality.
[0057] To minimize the change in optical output power per channel,
it is preferable that the power of light output from the light
source 52 be substantially equal to the power of the optical signal
cut off and that the wavelength of light output from the light
source 52 be included in the gain band of each optical amplifier
16.
[0058] The on/off operation of the light source 52 in accordance
with the predetermined rule by the control circuit 54 is performed
according to a control signal from a channel number monitor.
Specific embodiments of the channel number monitor will be
hereinafter described.
[0059] Further, it is preferable that the wavelength of light
output from the light source 52 be different from the wavelength of
the optical signal in each operative channel, so as to ensure a
normal operation of each operative channel. For example, the
wavelength of light output from the light source 52 is the same as
the wavelength of the optical signal cut off or the same as a
wavelength for an inoperative channel.
[0060] The light output from the light source 52 may be modulated
light or unmodulated continuous wave light (CW light). In the case
of modulating the light output from the light source 52 by a
modulating signal, information on the channel cut off can be
transmitted to each optical amplifier 16 or to the second terminal
station 4' by the modulating signal.
[0061] In the preferred embodiment shown in FIG. 7, the light
source 52 is dedicated for compensation such that the total power
of WDM signal light to be obtained becomes substantially constant.
Alternatively, any one of the optical senders 8' (#1 to #n) in an
inoperative channel may be used in place of the light source
52.
[0062] While the single light source 52 is used in the preferred
embodiment shown in FIG. 7, a plurality of light sources may be
used to cope with breaking of plural channels.
[0063] FIG. 8 is a block diagram showing a second preferred
embodiment of the optical transmission device according to the
present invention. In contrast with the first preferred embodiment
shown in FIG. 7, the second preferred embodiment further includes a
level adjusting unit 56 between the light source 52 and an input
port of the optical multiplexer 14. The level adjusting unit 56 is
provided by an optical attenuator having a variable attenuation or
an optical amplifier having a variable gain. The number of WDM
channels cut off can be detected according to a control signal to
be supplied to the control circuit 54. Accordingly, by adjusting
the level adjusting unit 56 according to the result of the above
detection, the total power of WDM signal light can be maintained
constant irrespective of the number of channels cut off, and the
optical output power per channel is prevented from exceeding an
allowable range of transmitted light output. While the level
adjusting unit 56 is provided independently of the light source 52
in this preferred embodiment, a laser diode capable of outputting
light having a controlled power according to a drive current may be
used as the light source 52, and the drive current may be adjusted
to thereby control the power of the light to be added to the WDM
signal light.
[0064] FIG. 9 is a block diagram showing a third preferred
embodiment of the optical transmission device according to the
present invention. In this preferred embodiment, any light source
dedicated for compensation such that the total power of WDM signal
light to be obtained becomes constant is not used, but a control
unit 58 is used to switch on or off light supplied from an external
light source and add the external light to the WDM signal light in
the optical multiplexer 14. More specifically, when the operative
channels are in normal operation, the external light is switched
off by the control unit 58, whereas when any one of the operative
channels is cut off, the external light is switched on by the
control unit 58 and added to the WDM signal light. Accordingly, the
total output power of the WDM signal light to be obtained can be
maintained always constant, and the optical output power per
channel can be made fall within an allowable range of transmitted
light output. As a result, there is no possibility of degradation
in transmission quality in the case that the number of WDM channels
is changed.
[0065] While the control unit 58 switches on or off the external
light in this preferred embodiment, the control unit 58 may be
modified to adjust the power of the external light and add the
external light to the WDM signal light on the basis of the level
adjusting unit 56 in the second preferred embodiment shown in FIG.
8. By applying such level adjustment to at least one channel, the
total output power of WDM signal light can be maintained constant
even when optical signals in plural channels are cut off. Further,
even in the case that the level adjustment is not carried out, it
is possible to cope with breaking of plural channels by applying
the compensating means according to the present invention to plural
channels, preferably, all the channels.
[0066] While the optical transmission device is applied to the
first terminal station 2' of the system shown in FIG. 2 in each of
the first to third preferred embodiments, the device may be applied
to the first terminal station 2 of the system shown in FIG. 1. In
this case, the wavelength converters 12 (#1 to #n) and another
necessary wavelength converter are provided on the input side of
the optical multiplexer 14.
[0067] FIG. 10 is a block diagram showing a preferred embodiment of
the channel number monitor for monitoring the number of WDM
channels cut off. A postamplifier 60 for amplifying WDM signal
light is provided at the output port of the optical multiplexer 14.
The WDM signal light amplified by the postamplifier 60 is branched
by an optical coupler 62, and resultant branch light is supplied to
a channel number monitor 64. The channel number monitor 64 performs
monitoring on the number of WDM channels cut off or the like
according to the input branch light. The result of monitoring is
output as the above-mentioned control signal.
[0068] According to this preferred embodiment, the monitoring is
performed on the output side of the optical multiplexer 14.
Accordingly, it is possible to detect a signal break due to device
failure, connector separation, package separation, etc. both on the
input side and on the output side of the optical multiplexer 14.
Further, since the WDM signal light obtained after wavelength
division multiplexing is supplied to the channel number monitor 64,
a signal break per channel can be detected by using a spectrum
analyzer in the monitor 64.
[0069] FIG. 11 is a block diagram showing another preferred
embodiment of the channel number monitor applicable to the present
invention. In this preferred embodiment, an optical coupler 66 is
provided at each input port of the optical multiplexer 14 to branch
an optical signal in each channel. A resultant branch optical
signal is supplied directly to a channel number monitor 64.
[0070] According to this preferred embodiment, the optical signals
before wavelength division multiplexing are supplied directly to
the monitor 64. Accordingly, a signal break in a specific channel
can be quickly detected without use of a spectrum analyzer.
Further, a circuit configuration from break detection to control
can be simplified, thereby achieving a high response speed.
[0071] FIG. 12 is a block diagram showing a fourth preferred
embodiment of the optical transmission device according to the
present invention. In this preferred embodiment, the optical
transmission device is applied to the first terminal station 2 of
the system shown in FIG. 1. In this preferred embodiment, an E/O
converter (electro/optical converter) 68 included in a wavelength
converter 12 (#i) in an inoperative channel, of the plural
wavelength converters 12 (#1 to #n) included in the transponder 10
is switched on or off by a control circuit 54. Alternatively, an
E/O converter in a wavelength converter in any channel where an
input optical signal is cut off rather than the E/O converter 68 in
the inoperative channel may be switched on or off by the control
circuit 54.
[0072] According to this preferred embodiment, the compensation
such that the total power of WDM signal light to be obtained
becomes constant can be performed by using the E/O converter in the
existing wavelength converter, thereby achieving the object of the
present invention with a simple configuration.
[0073] While the detection of breaking of an optical signal in a
certain one of the WDM channels can be performed by using the
channel number monitor 64 shown in FIG. 10 or 11, the breaking of
an optical signal may be detected by the following
configurations.
[0074] FIGS. 13A and 13B show preferred embodiments of the
wavelength converter for the detection of a signal break. In the
preferred embodiment shown in FIG. 13A, the wavelength converter 12
includes an O/E converter (opto/electrical converter) 70 for
converting an original optical signal supplied into an electrical
signal and an E/O converter 68 for converting the electrical signal
output from the O/E converter 70 into an optical signal. When a
signal break is detected in the E/O converter 68, the E/O converter
68 immediately emits steady light. That is, both the detection of a
signal break and the output of compensating light are completed in
the E/O converter 68. In the preferred embodiment shown in FIG.
13B, a break of input of an optical signal is detected in the O/E
converter 70. When a signal break is detected according to the
result of detection in the O/E converter 70, the E/O converter 68
emits steady light. According to the preferred embodiment shown in
FIG. 13A or 13B, the configuration of the existing wavelength
converter can be used without any changes, so that the compensation
can be performed with a simple configuration at a high response
speed.
[0075] In the case that ALC for maintaining the total output level
in each optical amplifier 16 constant is carried out in each of the
systems shown in FIGS. 1 and 2, the range of variation in optical
output level per channel due to a signal break is dependent on the
number of operative channels. For example, as shown in Table 1
below, when the number of operative channels changes from 2 to 1,
an optical output level of 3.01 dB is increased. In contrast
therewith, when the number of operative changes from 8 to 7, a
change in optical output as small as 0.58 dB.
1TABLE 1 Number of channels Number of channels Change in before
change after change optical output power 2 1 3.01 dB/ch. up 3 2
1.76 dB/ch. up 4 3 1.25 dB/ch. up 5 4 0.97 dB/ch. up 6 5 0.79
dB/ch. up 7 6 0.67 dB/ch. up 8 7 0.58 dB/ch. up
[0076] Accordingly, in the case that a part of the plural optical
senders 8 (#1 to #n) in the system shown in FIG. 1 is used for
signal transmission, or in the case that a part of the plural
optical senders 8' (#1 to #n) in the system shown in FIG. 2 is used
for signal transmission, a part or the whole of the remaining
optical senders is preferably operated to emit steady light,
thereby reducing the range of variation in optical output level per
channel in the case of a signal break in any operative channel.
[0077] To reduce the range of variation in optical output level per
channel, a dedicated light source rather than the optical sender in
an inoperative channel may be used. This will be described with
reference to FIG. 14.
[0078] FIG. 14 is a block diagram showing a fifth preferred
embodiment of the optical transmission device according to the
present invention. In this preferred embodiment, a light source 52
is connected to one of the input ports of the optical multiplexer
14. The light source 52 outputs light having a predetermined
wavelength. The light output from the light source 52 is added to
WDM signal light in the optical multiplexer 14. The wavelength of
the light output from the light source 52 is included in the gain
band of each optical amplifier 16, and set different from the
wavelength of an optical signal in each operative channel. While
the single light source 52 is shown in FIG. 14, a plurality of
light sources may be used. To minimize the range of variation in
optical output level per channel, the number of light sources 52 is
preferably increased.
[0079] FIG. 15 is a block diagram showing a sixth preferred
embodiment of the optical transmission device according to the
present invention. In this preferred embodiment, light supplied
from an external light source is added to WDM signal light in the
optical multiplexer 14, so as to reduce the range of variation in
optical output level per channel.
[0080] The optical transmission device shown in each of FIGS. 14
and 15 is applicable to the first terminal station 2' in the system
in FIG. 2, and applicable also to the input side of the wavelength
converters 12 (#1 to #n) in the system shown in FIG. 1.
[0081] In the preferred embodiment shown in FIG. 14, the power of
the light output from the light source 52 may be constant or may be
adjustable by use of the level adjusting unit 56 shown in FIG.
8.
[0082] Further, in the preferred embodiment shown in FIG. 15, the
control unit 58 shown in FIG. 9 may be applied to thereby make the
power of the external light adjustable.
[0083] FIG. 16 is a block diagram showing a seventh preferred
embodiment of the optical transmission device according to the
present invention. In this preferred embodiment, a wavelength
converter 12 (#i) in an inoperative channel is used to operate its
E/O converter 68 to emit steady light like the preferred embodiment
shown in FIG. 12. According also to this preferred embodiment, the
range of variation in optical output level per channel can be
reduced in the case of a signal break in any operative channel in
accordance with the principles described above with reference to
Table 1.
[0084] In carrying out the present invention, the power of the
compensating light is preferably controlled so that the output
level of each optical amplifier per channel becomes constant.
[0085] As described above, according to the present invention, it
is possible to provide an optical transmission device and an
optical communication system which can eliminate the possibility of
deterioration in transmission quality in the case that the number
of wavelength division multiplexed (WDM) channels is changed.
[0086] The present invention is not limited to the details of the
above described preferred embodiments. The scope of the invention
is defined by the appended claims and all changes and modifications
as fall within the equivalence of the scope of the claims are
therefore to be embraced by the invention.
* * * * *